Channel Strain Formation in Vertical Transport FETS with Dummy Stressor Materials

ABSTRACT

Techniques for forming VTFET devices with tensile- and compressively-strained channels using dummy stressor materials are provided. In one aspect, a method of forming a VTFET device includes: patterning fins in a wafer; forming bottom source and drains at a base of the fins; forming bottom spacers on the bottom source and drains; growing at least one dummy stressor material along sidewalls of the fins above the bottom spacers configured to induce strain in the fins; surrounding the fins with a rigid fill material; removing the at least one dummy stressor material to form gate trenches in the rigid fill material while maintaining the strain in the fins by the rigid fill material; forming replacement gate stacks in the gate trenches; forming top spacers on the replacement gate stacks; and forming top source and drains over the top spacers at tops of the fins. A VTFET device is also provided.

FIELD OF THE INVENTION

The present invention relates to vertical transport field-effecttransistor (VTFET) devices, and more particularly, to techniques forforming VTFET devices with tensile- and compressively-strained channelsusing dummy stressor materials.

BACKGROUND OF THE INVENTION

Vertical transport field-effect transistor (VTFET) devices havepotential advantages over conventional FinFETs in terms of density,power consumption, and integration. However, the lack of strain in thechannel of VTFET devices makes it difficult to compete with other deviceoptions in terms of performance.

Forming channel strain in VTFET devices is extremely challenging. Thechallenge is due to the free-standing fin channel-based structure of theVTFET devices which makes controlling strain in the vertical fin channeldifficult.

Accordingly, techniques for forming (compressive and tensile) strain inVTFET devices would be desirable.

SUMMARY OF THE INVENTION

The present invention provides techniques for forming vertical transportfield-effect transistor (VTFET) devices with tensile- andcompressively-strained channels using dummy stressor materials. In oneaspect of the invention, a method of forming a VTFET device is provided.The method includes: patterning fins in a wafer; forming bottom sourceand drains at a base of the fins; forming bottom spacers on the bottomsource and drains; growing at least one dummy stressor material alongsidewalls of the fins above the bottom spacers configured to inducestrain in the fins; surrounding the fins with a rigid fill material;removing the at least one dummy stressor material to form gate trenchesin the rigid fill material while maintaining the strain in the fins bythe rigid fill material; forming replacement gate stacks in the gatetrenches; forming top spacers on the replacement gate stacks; andforming top source and drains over the top spacers at tops of the fins.

In another aspect of the invention, a VTFET device is provided. TheVTFET device includes: fins patterned in a wafer; bottom source anddrains at a base of the fins; bottom spacers on the bottom source anddrains; gate stacks along sidewalls of the fins above the bottomspacers, wherein strain is present in the fins; a rigid fill materialsurrounding the fins and the gate stacks; top spacers on the replacementgate stacks; and top source and drains over the top spacers at tops ofthe fins.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional diagram illustrating fin hardmasks havingbeen used to pattern a plurality of (nFET/pFET) fins in a wafer,(nFET/pFET) bottom source and drains having been formed at the base ofthe nFET and pFET fins, bottom spacers having been formed on the nFETand pFET bottom source and drains, and a (first) liner having beenformed covering the pFET fins according to an embodiment of the presentinvention;

FIG. 2 is a cross-sectional diagram illustrating a dummy (tensile)stressor material having been grown on sidewalls of the nFET fins abovethe bottom spacers according to an embodiment of the present invention;

FIG. 3 is a cross-sectional diagram illustrating the first liner havingbeen removed from the pFET fins, and a (second) liner having been formedcovering the tensile dummy stressor material on the nFET fins accordingto an embodiment of the present invention;

FIG. 4 is a cross-sectional diagram illustrating a (compressive) dummystressor material having been grown on the exposed sidewalls of the pFETfins above the bottom spacers according to an embodiment of the presentinvention;

FIG. 5 is a top-down diagram illustrating a fin cut having beenperformed to remove portions of the fins from fin-cut regions dividingthe fins into individual nFET and pFET devices according to anembodiment of the present invention;

FIG. 6 is a cross-sectional diagram illustrating a thin liner havingbeen deposited onto the fins, over the tensile/compressive dummystressor materials, and a rigid fill material having been deposited overthe liner, surrounding the fins according to an embodiment of thepresent invention;

FIG. 7 is a top-down diagram illustrating how the rigid fill material ispresent surrounding the fins, and is in direct physical contact with theends of the fins in the fin cut regions according to an embodiment ofthe present invention;

FIG. 8 is a cross-sectional diagram illustrating the rigid fill materialand the liner having been recessed to expose the tops of thetensile/compressive dummy stressor materials according to an embodimentof the present invention;

FIG. 9 is a cross-sectional diagram illustrating the tensile dummystressor material having been removed selective to rigid fill materialand liner in the nFET devices, forming nFET gate trenches in the rigidfill material according to an embodiment of the present invention;

FIG. 10 is a cross-sectional diagram illustrating replacement gatestacks having been formed in the nFET gate trenches surrounding each ofthe nFET fins according to an embodiment of the present invention;

FIG. 11 is a cross-sectional diagram illustrating the compressive dummystressor material having been removed selective to rigid fill materialand liner in the pFET devices, forming pFET gate trenches in the rigidfill material according to an embodiment of the present invention;

FIG. 12 is a cross-sectional diagram illustrating replacement gatestacks having been formed in the pFET gate trenches surrounding each ofthe pFET fins according to an embodiment of the present invention;

FIG. 13 is a cross-sectional diagram illustrating the replacement gatestacks having been recessed to expose the tops of the fins according toan embodiment of the present invention;

FIG. 14 is a cross-sectional diagram illustrating top spacers havingbeen formed on the gate stacks according to an embodiment of the presentinvention; and

FIG. 15 is a cross-sectional diagram illustrating the fin hardmaskshaving been removed, and nFET/pFET top source and drains having beenformed over the top spacers at the tops of the nFET and pFET finsaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Channel strain can enhance field-effect transistor (FET) deviceperformance. For instance, compressive channel strain in p-channel FET(pFET) devices increases the hole mobility, whereas tensile channelstrain in n-channel FET (nFET) devices increases electron mobility.However, due to the free-standing nature of the fin channel of avertical transport field-effect transistor (VTFET) device, formingchannel strain is difficult. Namely, as opposed to planar complementarymetal-oxide-semiconductor (CMOS) devices, VTFET devices are orientedwith a vertical fin channel disposed on a bottom source/drain and a topsource/drain disposed on the fin channel.

Advantageously, provided herein are techniques for forming (compressiveand tensile) channel strain in VTFET devices which improves the electronand hole mobility. As will be described in detail below, dummy stressormaterials (which also double as a dummy gate) are used to induce strainin the vertical fin channel, and a rigid material is used at the finedge to preserve the strain after the dummy stressor materials areremoved. The term “dummy” material as used herein refers to asacrificial material that is placed for a given purpose at one point inthe process, and then later removed. A dummy gate is employed in agate-last process where processing conditions such as elevatedtemperatures can damage the gate stack materials. For instance, high-κmetal gate stack materials are particularly susceptible to damage fromthe high temperatures often employed during dopant drive-in andactivation anneals. Thus, a dummy gate can be employed early on in theprocess to permit formation of the source and drains, etc. and allassociated high-temperature processing to be performed. At the end ofthe process, the dummy gates are removed and replaced with a‘replacement’ metal gate. This process may also be referred to herein asa replacement metal gate or RMG process. Similarly, the dummy(gate/stressor) material will be used to induce strain in the verticalfin channel, which is then retained using a rigid material at the finsedge, after which, dummy/sacrificial stressor material is removed.

An exemplary methodology for forming a VTFET device in accordance withthe present techniques is now described by way of reference to FIGS.1-15. As shown in FIG. 1, the process begins with patterning a pluralityof fins 106 in a wafer 102. According to an exemplary embodiment, wafer102 is a bulk semiconductor wafer, such as a bulk silicon (Si), bulkgermanium (Ge), bulk silicon germanium (SiGe) and/or bulk III-Vsemiconductor wafer. Alternatively, wafer 102 can be asemiconductor-on-insulator (SOI) wafer. A SOI wafer includes a SOI layerseparated from an underlying substrate by a buried insulator. When theburied insulator is an oxide it is referred to herein as a buried oxideor BOX. The SOI layer can include any suitable semiconductor, such asSi, Ge, SiGe, and/or a III-V semiconductor.

To pattern the fins 106, a plurality of fin hardmasks 104 are patternedon the substrate 102 marking the footprint and location of the fins 106.Suitable hardmask materials include, but are not limited to, nitridematerials such as silicon nitride (SiN) and/or silicon oxynitride(SiON). Any suitable lithography and etching technique may be used topattern the fin hardmasks 104. For instance, direct patterning orsidewall image transfer (SIT) process may be employed. Advantageously,SIT (also referred to as self-aligned double patterning (SADP)) can beused to pattern fins at a sub-lithographic pitch, i.e., a pitch smallerthan what is achievable using direct lithographic patterning. A processwherein the SIT steps are repeated is referred to as self-alignedquadruple patterning (SAQP). The SAQP process is described, for example,in Fang et al., “SAQP Pitch walk metrology using single targetmetrology,” Proc. of SPIE, vol. 10145 (March 2017) (7 total pages), thecontents of which are incorporated by reference as if fully set forthherein. A directional (anisotropic) etching process such as reactive ionetching (RIE) is then used to pattern the fins 106 in the wafer 102using the fin hardmasks 104.

As shown in FIG. 1, one or more of fins 106 will serve as the verticalfin channels for an nFET device(s) and another one or more of fins 106will serve as the vertical fin channels for a pFET device(s). Asprovided above, it is desirable to have a tensile channel strain in nFETdevices for enhanced electron mobility, and compressive strain in pFETdevices for enhanced hole mobility. To illustrate the effectiveness ofpresent techniques for inducing both tensile and compressive channelstrain, in the present example both nFET and pFET VTFET devices will beformed concurrently on the same wafer. However, the same processesdescribed herein can be implemented to form only nFET or pFET VTFETdevices, if so desired.

Bottom source and drains 108 and 110 are formed at the base of the nFETand pFET fins 106, respectively. According to an exemplary embodiment,the bottom source and drains 108 and 110 are formed from an in-situdoped (e.g., during growth) or ex-situ doped (e.g., by ion implantation)epitaxial material. By way of example only, suitable source and drainmaterials for the nFET bottom source and drains 108 include, but are notlimited to, phosphorous-doped Si (Si:P). Suitable materials for the pFETbottom source and drains 110 include, but are not limited to,boron-doped SiGe (SiGe:B).

To isolate the nFET bottom source and drains 108 from the pFET bottomsource and drains 110, a shallow trench isolation (STI) region 112 isformed in the wafer 102. In general, STI involves patterning a trench inthe wafer 102 and then filling the trench with an insulator such as anoxide (also referred to herein as an “STI oxide”).

Bottom spacers 114 are then formed on the bottom source and drains 108and 110. See FIG. 1. Suitable materials for the bottom spacers 114include, but are not limited to, oxide spacer materials such as SiO₂and/or silicon oxycarbide (SiOC) and/or nitride spacer materials such asSiN and/or silicon-boron-nitride (SiBN). The bottom spacers 114 willserve to offset the gates of the device (see below) from the bottomsource and drains 108 and 110.

According to an exemplary embodiment, the bottom spacers 114 are formedusing a directional deposition process whereby the spacer material isdeposited onto the bottom source and drains 108 and 110 and fins 106with a greater amount of the material being deposited on horizontalsurfaces (including on top of the bottom source and drains 108 and 110in between the fins 106), as compared to vertical surfaces (such asalongside the fins 106). Thus, when an etch is used on the spacermaterial, the timing of the etch needed to remove the spacer materialfrom the vertical surfaces will leave the bottom spacers 114 shown inFIG. 1 on the bottom source and drains 108 and 110 since a greateramount of the spacer material was deposited on the bottom source anddrains 108 and 110. By way of example only, a high density plasma (HDP)CVD or physical vapor deposition (PVD) process can be used fordirectional film deposition, and an oxide- or nitride-selective(depending on the spacer material) isotropic etch can be used to removethe (thinner) spacer material deposited onto the vertical surfaces.

As will be described in detail below, the process for inducing channelstrain will involve protecting the fins of one device while adummy/sacrificial stressor material is grown on the other, and thenreversing the process to grow a different dummy stressor material on thefins of the other device. In the present example, the dummy stressormaterial will arbitrarily be grown on nFET fins 106 first. The pFET fins106 to be protected during this process. To do so, a liner 116 is formedcovering the pFET fins 106. According to an exemplary embodiment,placement of the liner 116 over the pFET fins 106 can be accomplished byfirst blanket depositing the liner 116 over all of the (nFET and pFET)fins 106, masking the liner 116 over the pFET fins 106 (e.g., using astandard block mask—not shown) and then etching/removing the liner 116from the nFET fins. As such, liner 116 needs to be formed from amaterial that can be etched selective to the bottom spacers 114. By wayof example only, when the bottom spacers 114 are formed from an oxidematerial (e.g., SiO₂ and/or SiOC) then the liner 116 can be formed froma nitride material (e.g., SiN and/or SiBN). Conversely, when the bottomspacers 114 are formed from a nitride material (e.g., SiN and/or SiBN)then the liner 116 can be formed from an oxide material (e.g., SiO₂and/or SiOC).

With the liner 116 in place over/protecting the pFET fins 106, a dummy(tensile) stressor material 202 is grown on sidewalls of the nFET fins106 above the bottom spacers 114. See FIG. 2. To induce tensile strainin the nFET fins 106, the dummy stressor material 202 generally shouldhave a greater lattice constant than the fins 106. Preferably, the dummystressor material 202 has a lattice constant of from about 1% to about2%, and ranges therebetween, greater than the lattice constant of thefins 106. For instance, according to an exemplary embodiment, the wafer102/fins 106 are Si. In that case, a suitable dummy stressor material202 is epitaxial SiGe grown on the exposed sidewalls of the nFET fins106 which (having a lattice constant that is from about 1% to about 2%,and ranges therebetween, greater than Si) will apply tensile strain tothe nFET fins 106. Preferably, the epitaxial SiGe, when used as dummystressor material 202, has a germanium (Ge) percentage of less thanabout 20% to avoid forming defects. Other materials having a greaterlattice constant than Si include, but are not limited to, epitaxialIII-V materials (i.e., materials containing at least one Group IIIelement and at least one Group V element from the periodic table). Asshown by arrows 204 in FIG. 2, the dummy stressor material 202 inducestensile strain in the nFET fins 106.

Following formation of the dummy stressor material 202 on the nFET fins106, the liner 116 is removed from the pFET fins 106. The process isthen repeated for the pFET fins 106 except a compressive dummy stressormaterial is used. For instance, as shown in FIG. 3, a liner 302 isformed covering the dummy stressor material 202 on nFET fins 106. Therequirements for the liner 302 are the same as that of liner 116 (i.e.,liner 302 needs to be selectively etched vis-à-vis the bottom spacers114). Suitable liner materials that provide this etch selectivity wereprovided above. According to an exemplary embodiment, the liner 302 isformed from the same material as liner 116. Further, as described above,the liner 302 can be formed selectively covering the dummy stressormaterial 202 on nFET fins 106 by first blanket depositing the liner 302over all of the (nFET and pFET) fins 106, masking the liner 302 over thedummy stressor material 202 on the nFET fins 106 (e.g., using a standardblock mask—not shown) and then etching/removing the liner 302 from thepFET fins.

With the liner 302 in place over/protecting the dummy stressor material202 on the nFET fins 106, a different (compressive) dummy stressormaterial 402 is then grown on the exposed sidewalls of the pFET fins 106above the bottom spacers 114. See FIG. 4. To induce compressive strainin the pFET fins 106, the dummy stressor material 402 generally shouldhave a smaller lattice constant than the fins 106. Preferably, the dummystressor material 402 has a lattice constant of from about 1% to about2%, and ranges therebetween, less than the lattice constant of the fins106. For instance, according to an exemplary embodiment, the wafer102/fins 106 are Si. In that case, a suitable dummy stressor material402 is epitaxial silicon carbide (SiC) grown on the exposed sidewalls ofthe pFET fins 106 which (having a lattice constant that is from about 1%to about 2%, and ranges therebetween, less than Si) will applycompressive strain to the pFET fins 106. Preferably, the epitaxial SiC,when used as dummy stressor material 402, has a carbon (C) percentage ofless than about 2% to avoid forming defects. As shown by arrows 404 inFIG. 4, the dummy stressor material 402 induces compressive strain inthe pFET fins 106. Following formation of the dummy stressor material402 on the pFET fins 106, the liner 302 is removed from the nFET fins106.

A fin cut can next be performed to divide the fins 106 into sectionscorresponding to individual nFET and pFET devices. Namely, the processbegins (as shown in FIG. 1) with patterning what is often referred to asa ‘sea of fins’ in the wafer 102. Cuts are then made along the length ofthe fins to create multiple sections, each of which corresponds to annFET or pFET device. To best illustrate the fin cut process, referenceshifts in FIG. 5 to a top-down view of the device structure (e.g., fromviewpoint A—see FIG. 4). To look at it another way, the cross-sectionalview provided in FIG. 4 is that of a cut along line A-A′ in FIG. 5.

As shown in FIG. 5, portions of the fins 106 are removed from fin-cutregions dividing the fins 106 into individual nFET and pFET devices. Itis notable that, from a top-down perspective, the fins 106 themselvesare not visible. However, the individual fin hardmasks 104 can be seen,each of which corresponds to a fin 106 below. As shown in FIG. 5, thefins 106 are cut on both ends of each individual device. This is notablesince the strain imparted by the dummy stressor materials 202/402 willbe locked in (i.e., maintained) using a rigid fill material (see below)which is deposited around the fins 106 (and particularly in directcontact with the ends of the fins 106 in the fins cut regions). Thatway, when the dummy stressor materials 202/402 are removed, the rigidfill material will prevent relaxation of the strain.

Another notable feature illustrated in FIG. 5 is that individual devicescan be configured on the wafer 102 with one type of device (nFET orpFET) arranged in front of/behind the other. For instance, for thedevices highlighted with dashed box 502, the two adjacent devices alongthe line A-A′ are nFET devices (i.e., having nFET bottom/top source anddrains and tensile stressor material), whereas the other two devices indashed box 502 are pFET devices (i.e., having pFET bottom/top source anddrains and compressive stressor material). The opposite orientation ispresent in the devices to the right of dashed box 502, and so on.

It is further notable that, despite the fins 106 being cut in the fincut regions, the channel strain remains present via the dummy stressormaterials 202/402 along the sidewalls of the fins 106. Later, a rigidmaterial will be deposited surrounding the fins (including at the endsof the fins in the fin cut regions) to hold the stress, allowing thedummy stressor materials 202/402 to be removed.

Prior to placing the rigid material, a thin protective liner 602 isfirst deposited onto the fins 106, over the dummy stressor materials202/402 and fin hardmasks 104, and onto the bottom spacers 114. Theliner 602 serves as a barrier between the fins 106 and the rigidmaterial to avoid any possible charging effects as well as undesirablereaction. According to an exemplary embodiment, the protective liner 602is formed by depositing a suitable liner material using a conformaldeposition process such as a chemical vapor deposition (CVD) or atomiclayer deposition (ALD) to a thickness of from about 1 nanometer (nm) toabout 4 nm, and ranges therebetween. Suitable materials for theprotective liner 602 include, but are not limited to, oxide materialssuch as silicon dioxide (SiO₂) and/or silicon oxycarbide (SiOC).

Next, a rigid fill material 604 is deposited over the protective liner602, surrounding the fins 106. See FIG. 6. Suitable rigid fill materialsinclude, but are not limited to, nitride materials such as siliconnitride (SiN) and/or silicon oxycarbonitride (SiOCN). According to anexemplary embodiment, the rigid fill material 604 is deposited using CVDor ALD, filling the spaces between the fins 106. Excess rigid fillmaterial 604 is then removed down to the protective liner 602 using aprocess such as chemical-mechanical polishing (CMP). By “rigid” it ismeant that material 604 is stiff enough to securely anchor the ends ofthe fins 106 and prevents relaxation of the strain when the dummystressor materials 202/402 are later removed. For instance, a materialsuch as SiO₂ is too flexible to be used as a stress anchor, while SiNand/or SiOCN are preferred for their functionality.

Switching again briefly to a top-down view, FIG. 7 is depiction fromviewpoint B—see FIG. 6. To look at it another way, the cross-sectionalview provided in FIG. 6 is that of a cut along line B-B′ in FIG. 7.Notably, the rigid fill material 604 is present surrounding the fins106, and is in direct physical contact with the ends of the fins 106 (inthe fin cut regions—see above) to lock in the stress from the dummystressor materials 202/402. As will be described in detail below, thispermits dummy stressor materials 202/402 to later be removed andreplaced with a replacement metal gate without relaxing the channelstrain induced by the dummy stressor materials 202/402. Thus, ashighlighted above, the dummy stressor materials 202/402 also serve asdummy gates in the present devices.

The rigid fill material 604 and the liner 602 are then recessed toexpose the tops of the dummy stressor materials 202/402. See FIG. 8. Asprovided above, the protective liner 602 can be formed from an oxidematerial. Thus, by way of example only, recess of the rigid fillmaterial 604 and the protective liner 602 can be accomplished using anitride-selective etching process such as a nitride-selective RIE torecess the rigid fill material 604 down to the protective liner 602,followed by an oxide-selective etch (such as an oxide-selective wetetch) to clear the protective liner 602.

Recessing the rigid fill material 604 and the protective liner 602 willpermit the dummy stressor materials 202/402 (also acting as dummy gates)to be removed and replaced with a replacement metal gate. For instance,beginning arbitrarily with the nFET devices, dummy stressor material 202is removed selective to rigid fill material 604 and protective liner602, forming gate trenches 902 in the rigid fill material 604. See FIG.9. For instance, dummy stressor material 202 can be removed using aself-aligned directional fin RIE again. Since fin hardmask 104 ispresent, a directional RIE provides the same fin structure without dummystressor material 202. A wet etch process can also be used toselectively remove a SiGe or III-V material against an Si fin.

Liner 602 protects the underlying fins 106 during the removal of dummystressor material 202. After removal of dummy stressor material 202, theliner 602 can be removed from the gate trenches 902 (including from thesidewalls of the fins 106) using an oxide-selective etch (such as anoxide-selective wet etch). As highlighted above, the rigid fill material604 remains in contact with the ends of the fins 106 and serves tomaintain strain in the fins 106 after removal of the dummy stressormaterial 202.

Replacement gate stacks (i.e., a gate dielectric and a gate conductor)are then formed in the gate trenches 902 surrounding each of the nFETfins 106. See FIG. 10. To form the (nFET) replacement gate stacks, aninterfacial oxide 1002 is first formed on exposed (sidewall) surfaces ofthe nFET fins 106 by an oxidation process to a thickness for example offrom about 0.3 nanometers (nm) to about 5 nm, and ranges therebetween,e.g., about 1 nm. A gate dielectric 1004 is then deposited into the gatetrenches 902 over the interfacial oxide 1002 and bottom spacers 114.Preferably, a conformal deposition process such as CVD or ALD is usedsuch that the gate dielectric 1004 lines the gate trenches 902. A gateconductor 1006 is then deposited into the gate trenches 902 over thegate dielectric 1004, e.g., using a process such as CVD, ALD,electrochemical plating, etc.

According to an exemplary embodiment, the gate conductor 1006 is ann-type workfunction setting metal(s) and the gate dielectric 1004 is ahigh-κ dielectric. Suitable n-type workfunction setting metals include,but are not limited to, titanium nitride (TiN), tantalum nitride (TaN)and/or aluminum (Al)-containing alloys such as titanium aluminide(TiAl), titanium aluminum nitride (TiAlN), titanium aluminum carbide(TiAlC), tantalum aluminide (TaAl), tantalum aluminum nitride (TaAlN),and/or tantalum aluminum carbide (TaAlC).

The term “high-κ” as used herein refers to a material having a relativedielectric constant κ which is much higher than that of silicon dioxide(e.g., a dielectric constant κ=25 for hafnium oxide (HfO₂) rather than3.9 for SiO₂). Suitable high-κ gate dielectrics include, but are notlimited to, HfO₂ and/or lanthanum oxide (La₂O₃).

The process is then repeated for the pFET devices, where the dummystressor material 402 is removed and replaced with a metal gate having ap-type workfunction setting metal(s). Namely, as shown in FIG. 11 dummystressor material 402 is removed selective to rigid fill material 604and liner 602, forming gate trenches 1102 in the rigid fill material604. As above, dummy stressor material 402 can be removed using aself-aligned directional fin RIE. Since fin hardmask 104 is present, adirectional RIE provides the same fin structure without dummy stressormaterial 402.

Liner 602 protects the underlying fins 106 during the removal of dummystressor material 402. After removal of dummy stressor material 402, theliner 602 can be removed from the gate trenches 1102 (including from thesidewalls of the fins 106) using an oxide-selective etch (such as anoxide-selective wet etch). As highlighted above, the rigid fill material604 remains in contact with the ends of the fins 106 and serves tomaintain strain in the fins 106 after removal of the dummy stressormaterial 402.

Replacement gate stacks (i.e., a gate dielectric and a gate conductor)are then formed in the gate trenches 1102 surrounding each of the pFETfins 106. See FIG. 12. To form the (pFET) replacement gate stacks, aninterfacial oxide 1202 is first formed on exposed (sidewall) surfaces ofthe pFET fins 106 by an oxidation process to a thickness for example offrom about 0.3 nm to about 5 nm, and ranges therebetween, e.g., about 1nm. A gate dielectric 1204 is then deposited into the gate trenches 1102over the interfacial oxide 1202 and bottom spacers 114. Preferably, aconformal deposition process such as CVD or ALD is used such that thegate dielectric 1204 lines the gate trenches 1102. A gate conductor 1206is then deposited into the gate trenches 1102 over the gate dielectric1204, e.g., using a process such as CVD, ALD, electrochemical plating,etc.

According to an exemplary embodiment, the gate conductor 1206 is ap-type workfunction setting metal(s) and the gate dielectric 1204 is ahigh-κ dielectric. Suitable p-type p-type workfunction setting metalsinclude, but are not limited to, TiN, TaN, and tungsten (W). TiN and TaNare relatively thick (e.g., greater than about 2 nm) when used as p-typeworkfunction metals. However, very thin TiN or TaN layers (e.g., lessthan about 2 nm) may also be used beneath Al-containing alloys in n-typeworkfunction stacks to improve electrical properties such as gateleakage currents. Thus, there is some overlap in the exemplary n- andp-type workfunction metals given here and above. As provided above,suitable high-κ gate dielectrics include, but are not limited to, HfO₂and/or La₂O₃.

Next, following deposition of the gate stack materials, the replacementgate stacks are recessed to expose the tops of the fins 106. See FIG.13. This replacement gate recess enables formation of the top spacersand top source and drains at the tops of the fins 106.

Specifically, as shown in FIG. 14, top spacers 1402 are formed on thegate stacks (using, e.g., a directional deposition process—see above).Suitable materials for the top spacers 1402 include, but are not limitedto, oxide spacer materials such as SiO₂ and/or SiOC and/or nitridespacer materials such as SiN and/or SiBN. The top spacers 1402 willserve to offset the replacement gate stacks from the top source anddrains (see below).

The fin hardmasks 104 are then removed and top source and drains 1502and 1504 are formed over the top spacers 1402 at the tops of the nFETand pFET fins 106, respectively. See FIG. 15. According to an exemplaryembodiment, the top source and drains 1502 and 1504 are formed from anin-situ doped (e.g., during growth) or ex-situ doped (e.g., by ionimplantation) epitaxial material. By way of example only, suitablesource and drain materials for the nFET top source and drains 1502include, but are not limited to, Si:P. Suitable materials for the pFETtop source and drains 1504 include, but are not limited to, SiGe:B.

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope of the invention.

What is claimed is:
 1. A method of forming a vertical transportfield-effect transistor (VTFET) device, comprising the steps of:patterning fins in a wafer; forming bottom source and drains at a baseof the fins; forming bottom spacers on the bottom source and drains;growing at least one dummy stressor material along sidewalls of the finsabove the bottom spacers configured to induce strain in the fins;surrounding the fins with a rigid fill material; removing the at leastone dummy stressor material to form gate trenches in the rigid fillmaterial while maintaining the strain in the fins by the rigid fillmaterial; forming replacement gate stacks in the gate trenches; formingtop spacers on the replacement gate stacks; and forming top source anddrains over the top spacers at tops of the fins.
 2. The method of claim1, wherein the fins comprise n-channel FET (nFET) fins and p-channel FET(pFET) fins, and wherein the at least one dummy stressor materialcomprises a first dummy stressor material configured to induce tensilestrain in the nFET fins and a second dummy stressor material configuredto induce compressive strain in the pFET fins.
 3. The method of claim 2,wherein the first dummy stressor material has a lattice constant that isfrom about 1% to about 2%, and ranges therebetween, greater than alattice constant of the fins.
 4. The method of claim 3, wherein the finscomprise silicon (Si), and wherein the at least one dummy stressormaterial is selected from the group consisting of: silicon germanium(SiGe) and a III-V material.
 5. The method of claim 2, wherein thesecond dummy stressor material has a lattice constant that is from about1% to about 2%, and ranges therebetween, less than a lattice constant ofthe fins.
 6. The method of claim 5, wherein the fins comprise Si, andwherein the at least one dummy stressor material comprises siliconcarbide (SiC).
 7. The method of claim 2, further comprising the stepsof: forming a first liner selectively covering the pFET fins; growingthe first dummy stressor material on the nFET fins; removing the firstliner from the pFET fins; forming a second liner selectively coveringthe nFET fins; growing the second dummy stressor material on the pFETfins; and removing the second liner.
 8. The method of claim 7, whereinthe first liner and the second liner are each formed from a materialselected from the group consisting of: silicon nitride (SiN),silicon-boron-nitride (SiBN), silicon dioxide (SiO₂), silicon oxycarbide(SiOC), and combinations thereof.
 9. The method of claim 1, wherein therigid fill material is selected from the group consisting of: SiN,silicon oxycarbonitride (SiOCN), and combinations thereof.
 10. Themethod of claim 1, further comprising the step of: cutting the fins intosections after growing the at least one dummy stressor material alongsidewalls of the fins, such that the rigid fill material surrounding thefins is in contact with ends of the fins in fin cut regions between thesections.
 11. The method of claim 1, wherein the bottom source anddrains and the top source and drains each comprise a material selectedfrom the group consisting of: phosphorous-doped Si (Si:P) andboron-doped SiGe (SiGe:B).
 12. The method of claim 1, further comprisingthe step of: depositing a protective liner onto the fins over the atleast one dummy stressor material prior to surrounding the fins with therigid fill material.
 13. The method of claim 12, wherein the protectiveliner has a thickness of from about 1 nm to about 4 nm, and rangestherebetween.
 14. The method of claim 12, wherein the protective linercomprises a material selected from the group consisting of: SiO₂, SiOCand combinations thereof.
 15. The method of claim 1, wherein forming thereplacement gate stacks in the gate trenches comprises the steps of:forming an interfacial oxide on the sidewalls of the fins; depositing agate dielectric into the gate trenches over the interfacial oxide; anddepositing a gate conductor into the gate trenches over the gatedielectric.
 16. The method of claim 15, wherein the gate dielectriccomprises a high-κ gate dielectric selected from the group consistingof: hafnium oxide (HfO₂), lanthanum oxide (La₂O₃), and combinationsthereof.
 17. The method of claim 15, wherein the gate conductorcomprises a workfunction setting metal selected from the groupconsisting of: titanium nitride (TiN), tantalum nitride (TaN), aluminum(Al)-containing alloys, titanium aluminide (TiAl), titanium aluminumnitride (TiAlN), titanium aluminum carbide (TiAlC), tantalum aluminide(TaAl), tantalum aluminum nitride (TaAlN), tantalum aluminum carbide(TaAlC), tungsten (W), and combinations thereof.
 18. A VTFET device,comprising: fins patterned in a wafer; bottom source and drains at abase of the fins; bottom spacers on the bottom source and drains; gatestacks along sidewalls of the fins above the bottom spacers, whereinstrain is present in the fins; a rigid fill material surrounding thefins and the gate stacks; top spacers on the replacement gate stacks;and top source and drains over the top spacers at tops of the fins. 19.The VTFET device of claim 18, wherein the fins comprise nFET fins andpFET fins, and wherein tensile strain is present in the nFET fins andcompressive strain is present in the pFET fins.
 20. The VTFET device ofclaim 18, wherein each of the replacement gate stacks comprises: aninterfacial oxide on the sidewalls of the fins; a high-κ gate dielectricdisposed over the interfacial oxide; and a gate conductor disposed onthe gate dielectric, wherein the gate conductor comprises a workfunctionsetting metal.